The existing DBS setting in patients with GPi DBS, which aims to treat motor symptoms by a high-frequency stimulation, will be temporarily reprogramed to stimulate the NBM at a low frequency using the distal electrodes positioned in the vicinity of the NBM.This study consists of a three-day screening study of the NbM stimulation followed by a two-week, randomized, single-blind, crossover trial. Between the screening study and the crossover trial, one week of washout period will be applied to eliminate potential carryover effect from the previous stimulation.